The present invention relates to antenna feeds for use in linear or circularly polarised systems. Particularly, but not exclusively, the invention relates to dual polarity antenna feeds particularly suitable for use in linearly polarised systems operating at S-band frequencies (approximately in the range 2 to 3 GHz) and Ku-band frequencies (about 12 GHz).
Conventionally, horn antenna feeds are used as dual polarity offset parabolic antenna feeds for systems operating at S-band frequencies; however, dielectric lens antenna feeds (sometimes called polyrod lenses) may be used instead of horn antenna feeds because horn antenna feeds for use at S-band frequencies are relatively large. FIG. 1 shows a typical prior art dielectric lens antenna and FIG. 2 shows the corresponding symmetrical radiation beam pattern with a 10 dB half beamwidth of 42.5xc2x0. FIG. 3 shows a typical prior art corrugated horn antenna feed and FIG. 4 shows the corresponding symmetrical radiation pattern. The corrugated feed shown in FIG. 4 has a 35xc2x0 10 dB half beamwidth. The horn feed shown in FIG. 3 shows complete round corrugations with a constant feed angle xcex8 which results in the beam pattern of FIG. 4.
Dielectric lens antenna feeds have the advantage that they are physically smaller than horn antenna feeds but provide similar electrical performance.
The dielectric lens is made of solid plastic material typically by a plastic moulding process but this gives rise to manufacturing problems because the outside of the moulded lens cools quicker than the inside and premature removal from the mould before the plastics material has fully set can result in physical discontinuities in the lens, such as cavities, which reduce performance of the lens in the antenna feed. Merely waiting a much longer time for the plastics material to set reduces manufacturing throughput and increases the cost per unit item. This problem is exaggerated for lower frequency lenses which are physically larger in size.
It is an object of the present invention to provide an antenna feed which obviates or mitigates at least one of the above disadvantages.
According to a first aspect of the present invention there is provided an antenna feed for use in a system for receiving orthogonal linear or circularly polarised signals, the antenna feed comprising an antenna feed body for coupling to a waveguide housing, the feed body defining a central axis and having spaced arms extending radially outwardly from the central axis for receiving the polarised signals.
By virtue of the present invention an antenna feed of reduced weight can be manufactured. The manufacturing process is less expensive than for conventional dielectric lens feeds because less dielectric material is required. In addition, because there is less volume of material, the lens cools quicker in the centre thereby minimising discontinuities and providing improved lens quality. The throughput of manufactured lenses can be increased because of the reduced cooling time requirements.
Where the antenna feed is a dielectric lens, the feed body advantageously has a generally cruciform cross-section and comprises a central dielectric core co-axial with the central axis, and peripherally-spaced dielectric arms of the cross disposed around the core. The arms may be separated from each other by an air gap or the arms may be separated from each other by, for example, another dielectric material. It will be appreciated that the central core and the arms are preferably manufactured as a single unit, by moulding or machining, thus there is no join between the arms and the central core. Alternatively, the central core may be made of separate pieces which are subsequently joined together.
Where the antenna feed is a cross-type feed, the spaced arms may be in the form of corrugated radially extending portions, each portion having ridges extending therefrom in spaced parallel relation.
It will be appreciated that the feed body and the housing for the lens or for the cross-type feed may be moulded or cast as an integral unit. This may lead to a reduction in weight and cost. The antenna feed may be adjusted to receive polarised signals of different beamshapes by changing a feed angle of the antenna feed, by adjusting a) the height of corrugations, b) the spacing between the corrugations and c) the position of the corrugations along the z-axis.
As will be appreciated by persons skilled in the art, references herein and in the following description to a feed angle of a cross-type antenna feed are to an angle defined between the central axis of the waveguide housing and a plane defining a surface connecting the ridge of each arm of the cross.
According to a second aspect of the present invention, there is provided an antenna feed for use in a system for receiving orthogonal linearly or circularly polarised signals, the antenna feed comprising an antenna feed body for coupling to a waveguide housing, the feed body defining a central axis and having a plurality of spaced arm portions extending radially outwardly from the central axis on a respective radius for receiving the polarised signals, each arm portion being corrugated and having at least one element arranged transversely to its respective radius.
Preferably, the antenna feed is a cross-type feed.
Preferably, the element comprises a substantially straight ridge. The element may be disposed substantially perpendicularly to the respective arm portion radius.
Conveniently, each arm portion has two or more elements arranged in spaced parallel relation.
Alternatively, the at least one element may comprise two or more straight ridges, disposed adjacent to and at an angle from one another. Preferably, each element comprises three straight ridges. Conveniently, each arm portion has two or more elements arranged in spaced parallel relation.
Preferably, the antenna feed body is generally cylindrical. The antenna feed body may be tubular.
Conveniently, the corrugated arm portions extend radially outwardly from the antenna feed body. There may be four corrugated arm portions disposed around a circumference of the antenna feed body. The corrugated arm portions may be mutually perpendicularly disposed around the circumference of the antenna feed body. Preferably, first and second mutually opposed ones of said corrugated arm portions are disposed at a first feed angle, whilst third and fourth mutually opposed ones of said corrugated arm portions are disposed at a second feed angle. It will be appreciated by persons skilled in the art that the disposition of the corrugated arm portions at first and second feed angles allows the antenna feed to generate an elliptical beam shape and to receive polarised signals from an elliptical dish.
According to a third aspect of the present invention there is provided a method of receiving orthogonal linear or circularly polarised signals, the method including the steps of:
providing an antenna feed body defining a central axis and having spaced arms extending radially outwardly from the central axis for receiving the polarised signals;
coupling the antenna feed body to a waveguide housing;
arranging the antenna feed body in relation to an antenna so that, in use, the arms of the antenna feed body receive polarised signals reflected by the antenna and convey these signals to the waveguide housing.
According to a fourth aspect of the present invention there is provided an antenna system comprising:
a reflector antenna;
an antenna feed in accordance with the first aspect of the present invention; and
a waveguide housing for coupling to the antenna feed,
so that, in use, signals are reflected by the antenna, received by the antenna feed, and propagated along the waveguide housing.
It will be appreciated that the feed body may have any convenient shape, for example, the feed body may be generally circular, oval, square, or rectangular.
According to a fifth aspect of the present invention there is provided a low noise block (LNB) receiver for use with an antenna system, the LNB receiver comprising:
an antenna feed in accordance with the first aspect of the present invention;
a waveguide housing coupled to the antenna feed, the waveguide housing having probes disposed therein; and
a circuit board in electrical communication with the probes having an output for providing electrical signals corresponding to incoming polarised signals.